New York City Mayor Michael Bloomberg says a poison-laced letter sent to his office appears to be linked to his high-profile campaign for gun control.

Bloomberg spoke to reporters late Wednesday after traces of the deadly poison ricin was discovered on two letters sent to the mayor. He said at least one of the letters cited his work for tougher U.S. gun laws, but that he would not be deterred.

"It's not the first letter that was ever sent to anybody. In terms of why they've done it, I don't know, the letter obviously referred to our anti-gun efforts, but there's 12-thousand people that are going to be killed this year with guns and 19-thousand are going to commit suicide with guns and we are not going to walk away from those efforts."

Bloomberg was a prime organizer of Mayors Against Illegal Guns, a group formed in 2006 with a meeting of 15 mayors. The group now comprises more than 950 mayors from across the country.

The group stepped up its efforts this year after a lone gunman killed 26 people, most of them young children, in nearly Newtown, Connecticut. The organization backed its campaign with a $12 million advertising effort, financed in part from Bloomberg's personal fortune.

Despite the effort, opponents of any new restrictions on the right to own and carry firearms, including the influential National Rifle Association, have blocked all efforts to push new gun laws through the U.S. Congress this year.